1. Field of the Invention
This invention relates to semiconductor devices that are subject to hot carrier stress during their operation and, more particularly, to lateral diffused metal-oxide-semiconductor field effect transistors (LDMOS FETs) that are subject to hot carrier injection (HCI).
2. Discussion of the Related Art
Some MOSFET semiconductor devices, especially radio frequency (RF) LDMOS FETs of the type shown in FIG. 1, are operated at relatively high powers, which may cause energetic charged carriers (i.e., electrons, holes) to be injected from the drain into/through the gate oxide, a phenomenon known as HCI. Charge trapped in the gate oxide has several possible adverse effects including drift of the operating current (e.g., the quiescent drain current, IDq) and restriction of the voltage/current sweep range. In particular, trapped charge can cause degradation of the saturation current, transconductance, threshold voltage and on-resistance (RON). As a result, power capability decreases during the lifetime of RF amplifiers that employ such LDMOS FETS.
Thus, the stress induced by HCI is an important consideration in determining the reliability of semiconductor devices such as LDMOS FETS.
The problem is complicated by recent RF amplifier designs in which efficiency is improved by operating schemes that dynamically vary the drain bias; that is, the drain bias, instead of being maintained constant in time, is controllably varied according to a predetermined function (e.g., a probability density function).
A need remains in the art for a technique that predicts the effects of hot carrier stress under dynamic or variable drain bias conditions.